Relay double-spends, subject to anti-DOS

Allows network wallets and other clients to see transactions that respend
a prevout already spent in an unconfirmed transaction in this node's mempool.

Knowledge of an attempted double-spend is of interest to recipients of the
first spend.  In some cases, it will allow these recipients to withhold
goods or services upon being alerted of a double-spend that deprives them
of payment.

As before, respends are not added to the mempool.

Anti-Denial-of-Service-Attack provisions:
 - Use a bloom filter to relay only one respend per mempool prevout
 - Rate-limit respend relays to a default of 100 thousand bytes/minute
 - Define tx2.IsEquivalentTo(tx1): equality when scriptSigs are not considered
 - Do not relay these equivalent transactions

Remove an unused variable declaration in txmempool.cpp.

src/core.cpp

@@ -119,6 +119,22 @@ CTransaction& CTransaction::operator=(const CTransaction &tx) {
     return *this;
 }
 
+bool CTransaction::IsEquivalentTo(const CTransaction& tx) const
+{
+    if (nVersion   != tx.nVersion   ||
+        nLockTime  != tx.nLockTime  ||
+        vin.size() != tx.vin.size() ||
+        vout       != tx.vout)
+        return false;
+    for (unsigned int i = 0; i < vin.size(); i++)
+    {
+        if (vin[i].nSequence != tx.vin[i].nSequence ||
+            vin[i].prevout   != tx.vin[i].prevout)
+            return false;
+    }
+    return true;
+}
+
 int64_t CTransaction::GetValueOut() const
 {
     int64_t nValueOut = 0;